In 3GPP, which is an organization that sets standards for third generation mobile communications systems, a study collectively termed as LTE (Long Term Evolution) has been conducted for achieving a drastic improvement of transmission speed and reduction of transmission delay in a radio access network (RAN); and formulation of standard specifications of the elemental technologies related to the study has been in progress.
As shown in FIG. 4, the radio access network (E-UTRAN: Evolved Universal Terrestrial RAN) in an LTE mobile communication system is configured of a mobile station UE (User Equipment) and a radio base station eNB (E-UTRAN Node B). This system is configured in such a way that the mobile station UE and the radio base station eNB communicate with each other via a radio link (RL).
Moreover, each of the mobile station UE and the radio base station eNB is configured to terminate an RLC (Radio Link Control) sublayer, a MAC (Medium Access Control) sublayer and a physical (PHY: Physical) layer.
In addition, a transmitting-side apparatus (mobile station UE or radio base station eNB) is configured to perform RLC processing, MAC processing and PHY processing sequentially for data to be transmitted and then to transmit the data as radio signals from a radio unit.
Meanwhile, a receiving-side apparatus (mobile station UE or radio base station eNB) is configured to extract the transmitted data by performing PHY processing, MAC processing and RLC processing sequentially for the radio signals received by a radio unit.
Here, the data to be transmitted includes user data (U-plane data) generated by an application or the like used by the user, and control data (C-plane data) used in controlling the mobile communication system, such as RRC (Radio Resource Control) signaling and NAS (Non Access Stratum) signaling.
Moreover, this system is configured in such a way that RLC retransmission control processing is performed between the RLC sublayer of the transmitting-side apparatus and the RLC sublayer of the receiving-side apparatus, and that HARQ (Hybrid Automatic Repeat Request) retransmission control processing (MAC retransmission control processing) is performed between the MAC sublayer of the transmitting-side apparatus and the MAC sublayer of the receiving-side apparatus.
Here, in an IMT-2000 mobile communication system, a receiving-side apparatus is configured so that the MAC sublayer performs reordering processing for received RLC-PDUs (actually, MAC-PDUs to which the RLC-PDUs are mapped), and then transmits the RLC-PDUs to the RLC sublayer in the order of the sequence numbers, as shown in FIG. 1.
Accordingly, the RLC sublayer is configured to unconditionally determine that a packet loss has occurred, when receiving an RLC-data-PDU of a sequence number #4 before receiving an RLC-data-PDU (AMD-PDU in an IMT-2000 mobile communication system) of a sequence number #3, and to transmit a STATUS-PDU (NACK) that requests retransmission of the RLC-data-PDU of the sequence number #3, as shown in FIG. 2.    Non-Patent Document 1: RLC Specification of IMT-2000: 3GPP TS 25.322 V6.9.0 (2006-09)    Non-Patent Document 2: LTE Stage 2 Specification: 3GPP TS 36.300 V8.0.0 (2007-03)